1. Field of the Invention
The present invention relates to a process for preparing multi-layered coated articles and to the coated articles themselves. More particularly, the invention relates to a process for preparing a multi-layered coating comprising a basecoat and a topcoat in which the basecoat is aqueous-based and contains color pigments, and the topcoat is transparent.
2. Brief Description of the Prior Art
Metallic coatings are becoming increasingly popular, particularly for use in automobiles. These coatings contain a metallic pigment such as aluminum flake along with a polymeric binder. The coatings have a very lustrous appearance which can be enhanced by topcoating the metallic coating with a clear topcoat which is often organic solvent based. This system is commonly referred to as "clear-over-color" or "color plus clear".
In developing the clear-over-color systems in which the color coat contains metallic pigments, it has been found desirable to incorporate an additive in the basecoat composition which will help orient the metallic pigment such that it is parallel to the coated surface. Alignment in this fashion provides for the most pleasing appearance particularly with regard to "flop". By "flop" is meant the visual change in brightness or lightness of a metallic coating with the change in viewing angle, that is, a change from 90 through 180 degrees. The greater the change, that is, from a light to dark appearance, the better the flop. Flop is important because it accentuates the lines and curves of an automobile.
The additive also enables the basecoat to set or dry quickly so that an organic solvent base topcoat composition can be applied without the solvents in the topcoat attacking the basecoat, i.e., topcoat "striking in" to the basecoat. This "striking in" adversely affects alignment of the metallic pigment and the gloss of the topcoat. Although the basecoat could be first cured by heating before application of the topcoat, this is not favored by coatings users, particularly the automotive industry, who wish to apply the topcoat to the basecoat via a "wet-on-wet" application followed by a single heating step to cure the composite coating. Examples of suitable additives for incorporation into the basecoat are cellulose acetate butyrate and crosslinked polymeric microparticles (microgel) which are described in U.S. Pat. Nos. 3,639,147 and 4,220,679, respectively.
It has also been found in developing this invention that with regard to the appearance of metallic coatings, water-based basecoats provide for the brightest coating and the best flop. Color plus clear systems using water-based basecoats are described in U.S. Pat. No. 4,403,003. However, this reference discloses the use of microgel in the basecoat and this can pose a disadvantage. One of the most popular microgels are those described in U.S. Pat. No. 4,147,688. However, these microgels are prepared in organic solvent and are difficult to incorporate into a water-based system. Although there are microgels which are prepared in aqueous systems such as those described in British Patent No. 967,051 and U.S. Pat. No. 4,290,932, these aqueous-based microgels are difficult to prepare in high yields and can present stability problems when incorporated into paints.
Surprisingly, it has been found that certain water-dilutable polymeric binders can be used in the formulation of metallic pigment-containing basecoats which do not require the presence of crosslinked polymeric microparticles or cellulose acetate butyrate. Transparent topcoats can be applied to these basecoats via a wet-on-wet technique to provide for lustrous multi-layered coated articles with excellent flop. The invention is also applicable to basecoats without metallic pigmentation but with color pigments. The water-dilutable polymeric binders can be applied over both metallic portions and elastomeric portions of a substrate such as are commonly found on motor vehicle bodies thereby providing a universal basecoat. Also in a preferred embodiment of the invention, specific topcoats can be applied to the basecoat over both elastomeric and metallic parts providing a universal color plus clear coating system. Such universal coating system is particularly desirable in the automotive industry where the metallic parts of the motor vehicle are painted with one type of paint and the elastomeric parts such as bumpers and front end facia are painted with another type of paint which has to be more flexible than the paint for the metallic parts. The use of two types of paints is labor intensive and presents problems with color matching. The universal coatings of the present invention overcome these disadvantages.